A multi-level coarse-grain model to speed up CFD-DEM simulations
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The combination of Computational Fluid Dynamics and the Discrete Element Method (CFD-DEM) has proven a to be a viable tool for the analysis of granular flows. Especially for industrial facilities where experimental measurements are difficult to carry out, accurate simulations are essential to guide process and equipment design. However, the key advantage of the DEM, namely resolving individual particle contacts, is also the major shortcoming of the method since it comes with a high computational demand. As a consequence, the DEM part also determines the performance of a typical CFD-DEM solver.
The coarse-grain model of the DEM alleviates the hardware equirements, representing multiple particles by one coarse grain, thus effectively reducing the number of particles involved in the calculations. However, this approach fails for effects that intrinsically depend on particle size. For processes operating at multiple scales, we thus have developed a method which couples multiple coarse-grain levels
to adjust the resolution of the simulation as needed. The coupling is established by exchanging volumetric properties of the granular flow to impose proper boundary conditions in each sub-region. In this way it is possible to preserve the particulars of the granular system in spatially confined regions and at the same time benefit from the speedup of the coarse-grain model, where a lower level of detail is sufficient. On the CFD side, the mesh resolution can be chosen accordingly.
The method was validated by comparing the computed statistical properties of the multi-level coarse-grain model with the corresponding properties of the fully resolved reference system.